This invention relates in general to a coating system and, more specifically, to a system for immersion coating of drums.
Electrophotographic imaging members are well known. Typical electrophotographic imaging members include photosensitive members (photoreceptors) that are commonly utilized in electrophotographic (xerographic) processes in either a flexible belt or a rigid drum configuration. These electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photo generating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a member such as paper.
Electrophotographic imaging drums may be coated by many different techniques such as spraying coating or immersion (dip) coating. Dip coating is a coating method typically involving dipping a substrate in a coating solution and taking up the substrate. In dip coating, the coating thickness depends on the concentration of the coating material and the take-up speed, i.e., the speed of the substrate being lifted from the surface of the coating solution. It is known that the coating thickness generally increases with the coating material concentration and with the take-up speed.
The need for faster printing speed, e.g., up to about 75 pages per min and printing two pages side by side has lead to the development of long, large diameter drum substrates, instead of shorter small diameter drums. The larger diameter drums also provide more surface area around the periphery of the drum to locate large, space consuming development stations. For such requirements, the drum dimension can be 38 centimeters (15 inches) in diameter and 76 centimeters (30 inches) in length. As the size and weight of the substrate is increased, the problems involving in inserting and withdrawing the substrate from a coating vessel are compounded. Thus, for example, dip coating of large heavy hollow cylinders requires large quantities of coating liquid which can be wasteful if the coating liquid has a short pot life. Moreover, vibration or wobble during transport of a large heavy drum into and out of a coating liquid can cause undesirable coating defects.
Another technique for immersion coating comprises (a) positioning the substrate within a coating vessel to define a space between the vessel and the substrate and providing a downwardly inclined surface contiguous to the outer surface at the end region of the substrate; (b) filling at least a portion of the space with a coating solution; and (c) withdrawing the coating solution from the space, thereby depositing a layer of the coating solution on the substrate. This process is described in U.S. Pat. No. 5,616,365, the entire disclosure thereof being incorporated herein by reference. When this process is utilized for coating a large drum, e.g. 24 centimeters (9.5 inches) in diameter, in which coating fluid is withdrawn at the bottom to deposit a coating layer on the drum located in the center of a coating vessel, it can produce uniform and defect free coating for thin undercoating layers and thick charge transport layers. However, when drums are dip coated in essentially a closed environment using a minimum amount of coating fluid and without requiring precise motion control of potentially massive substrates over large distances as in the system described in U.S. Pat. No. 5,616,365, it has been found that substrate is not always centered when inserted into a coating vessel for a coating operation. Non-uniform coatings can be formed when the spacing between the outside surface of the drum being coated and the adjacent coating vessel wall is not uniform completely around the outer surface of the drum.
These defects are unacceptable for high printing quality requirements such as extremely uniform thickness and defect free coatings. Solutions to these coating problems are crucial for complex, advanced precision tolerance imaging systems.
U.S. Pat. No. 5,616,365 to Nealey, issued Apr. 1, 1997xe2x80x94A method is disclosed for coating a substrate having an end region including: (a) positioning the substrate within a coating vessel to define a space between the vessel and the substrate and providing a downwardly inclined surface contiguous to the outer surface at the end region of the substrate; (b) filling at least a portion of the space with a coating solution; and (c) withdrawing the coating solution from the space, thereby depositing a layer of the coating solution on the substrate.
U.S. Pat. No. 5,693,372 to Mistrater et al, issued Dec. 2, 1997xe2x80x94A process for dip coating drums comprising providing a drum having an outer surface to be coated, an upper end and a lower end, providing at least one coating vessel having a bottom, an open top and a cylindrically shaped vertical interior wall having a diameter greater than the diameter of the drum, flowing liquid coating material from the bottom of the vessel to the top of the vessel, immersing the drum in the flowing liquid coating material while maintaining the axis of the drum in a vertical orientation, maintaining the outer surface of the drum in a concentric relationship with the vertical interior wall of the cylindrical coating vessel while the drum is immersed in the coating material, the outer surface of the drum being radially spaced from the vertical interior wall of the cylindrical coating vessel, maintaining laminar flow motion of the coating material as it passes between the outer surface of the drum and the vertical interior wall of the vessel, maintaining the radial spacing between the outer surface of the drum and the inner surface of the vessel between about 2 millimeters and about 9 millimeters, and withdrawing the drum from the coating vessel.
U.S. Pat. No. 5,725,667 to Petropoulos et al, issued Mar., 10, 1998xe2x80x94There is disclosed a dip coating apparatus including: (a) a single coating vessel capable of containing a batch of substrates vertically positioned in the vessel, wherein there is absent vessel walls defining a separate compartment for each of the substrates; (b) a coating solution disposed in the vessel, wherein the solution is comprised of materials employed in a photosensitive member and including a solvent that gives off a solvent vapor; and (c) a solvent vapor uniformity control apparatus which minimizes any difference in solvent vapor concentration encountered by the batch of the substrates in the air adjacent the solution surface, thereby improving coating uniformity of the substrates.
U.S. Pat. No. 5,820,897 to Chambers et al, issued Oct. 13, 1998xe2x80x94This invention discloses a method of holding and transporting a hollow flexible belt throughout a coating process. The method includes placing an expandable insert into the hollow portion of a seamless flexible belt, and expanding the insert until it forms a chucking device with a protrusion on at least one end. A mechanical handling device is then attached to the protrusion, and will be used to move the chuck and the belt through the dipping process, as materials needed to produce a photosensitive device are deposited onto the surface of the belt, allowing it to be transformed into an organic photoreceptor. The chucking device and flexible belt are then removed from the mechanical handling device, the belt is cut to the desired width, and the chuck is removed from the inside of the photoreceptor.
In U.S. patent application Ser. No. 09/466,565, entitled xe2x80x9cIMMERSION COATING PROCESSxe2x80x9d, filed concurrently herewith in the names of Dinh et al., there is disclosed a process is disclosed for immersion coating of a substrate including positioning a substrate having a top and bottom within a coating vessel having an inner surface to define a space between the inner surface and the substrate, filling at least a portion of the space with a coating mixture; stopping the filling slightly below the top of the substrate, initiating removal of the coating mixture at a gradually increasing rate to a predetermined maximum flow rate in a short predetermined distance, and continuing removal of the coating mixture at substantially the predetermined maximum flow rate to deposit a layer of the coating mixture on the substrate. The aforementioned co-pending application is assigned to Xerox Corporation.
The entire disclosure of the above patent application is incorporated herein by reference.
It is, therefore, an object of the present invention to provide an improved immersion coating system that overcomes the above noted deficiencies.
It is another object of the present invention to provide an improved immersion coating system that forms uniform coatings.
It is still another object of the present invention to provide an improved immersion coating system that forms coatings on large cylindrical substrates.
It is yet another object of the present invention to provide an improved immersion coating system that more precisely centers cylinders within a cylindrical coating chamber to form uniform coatings during withdrawal of coating mixtures from the coating vessel.
It is another object of the present invention to provide an improved immersion coating system that allows smooth and even withdrawal of the coating solutions from a coating chamber.
It is still another object of the present invention to provide an improved layered electrostatographic imaging member whereby the interior surface of the object to be coated is protected from a coating solution by a suitable seal arrangement.
The foregoing objects and others are accomplished in accordance with this invention by providing a coating process comprising
providing an assembly comprising a hollow cylinder having an upper end and a lower end sandwiched between and in pressure contact with a first spacing device and a second spacing device, a hollow shaft coaxial with the cylinder connecting the first spacing device and the second spacing device,
mounting the assembly on a vertical rod which is concentric to and mounted within a cylindrical coating vessel having a top and bottom,
introducing coating liquid into the coating vessel adjacent to the bottom to immerse most of the cylinder, and
withdrawing the liquid from the coating vessel adjacent to the bottom to deposit a layer of the coating liquid on the cylinder.
This invention also comprises apparatus for carrying out the coating process of this invention.